Cosine function does not work

I have recently faced a silly problem using cosine (Y=cos(x)). I am trying to compute:

cos(ksi1)

where ksi1 equals to 2.8322 and is type double. I am getting only an error:

"Subscript indices must either be real positive integers or logicals."

What is funny that sin(ksi1) returns a value, 0.3044. Moreover, I can compute value cos(ksi1) using Pythagorean trigonometric identity.

It just eats my nerves. I am working with MATLAB R2013a.

Answer 1

[James Tursa]

You have inadvertently created a variable with the name "cos" that is shadowing the function "cos". Clear the variable, and pick a different variable name to use for the calculation you are doing to avoid the same problem in the future. E.g.,

>> which cos
built-in (C:\Program Files\MATLAB\R2015a\toolbox\matlab\elfun\@double\cos)  % double method
>> cos(2.8322)
ans =
   -0.9525
>> cos = 4 % <-- creating a variable named cos
cos =
     4
>> which cos
cos is a variable.
>> cos(2.8322)
Subscript indices must either be real positive integers or logicals. 
>> clear cos
>> cos(2.8322)
ans =
   -0.9525

[Szymon Wlodarczyk]

That was the problem. Thank you.

Answer 2

[mustafa ün]

execllent fro beginners